The present invention relates to the arts of nuclear medicine and diagnostic imaging. It finds particular application in conjunction with gamma or scintillation cameras and will be described with particular reference thereto. It is to be appreciated that the present invention is applicable to single photon emission computed tomography (SPECT), positron emission tomography (PET), whole body nuclear scans, and the detection of radiation for other applications.
Diagnostic nuclear imaging is used to study a radionuclide distribution in a subject. Typically, one or more radiopharmaceuticals or radioisotopes are injected into a subject. The radiopharmaceuticals are commonly injected into the subject""s blood stream for imaging the circulatory system or for imaging specific organs which absorb the injected radiopharmaceuticals. Gamma or scintillation camera detector heads, typically including collimators, are placed adjacent to a surface of the subject to monitor and record emitted radiation. Often, the detector heads are rotated or indexed around the subject to monitor the emitted radiation from a plurality of directions. The monitored radiation data from the multiplicity of directions is reconstructed into a three dimensional image representation of the radiopharmaceutical distribution within the subject.
One of the problems with this imaging technique is that photon absorption and scatter by portions of the subject between the emitting radionuclide and the camera heads distort the resultant image. One solution for compensating for photon attenuation is to assume uniform photon attenuation throughout the subject. That is, the subject is assumed to be completely homogeneous in terms of radiation attenuation with no distinction made for bone, soft tissue, lung, etc. This enables attenuation estimates to be made based on the surface contour of the subject. However, human subjects do not cause uniform radiation attenuation, especially in the chest.
In order to obtain more accurate radiation attenuation measurements, a direct measurement is made using transmission computed tomography techniques. In this technique, radiation is projected from a radiation source through the subject. Radiation that is not attenuated is received by detectors at the opposite side. The source and detectors are rotated to collect transmission data concurrently or sequentially with the emission data through a multiplicity of angles. This transmission data is reconstructed into an image representation using conventional tomography algorithms. The radiation attenuation properties of the subject from the transmission computed tomography image are used to correct or compensate for radiation attenuation in the emission data.
Dual and triple head gamma cameras are now equipped for simultaneous collection of transmission and emission data in order to provide enhanced PET and SPECT attenuation correction. Typically, the transmission device consists of a collimated radioactive line source or a point source mounted for movement along a shielded cylinder. The cylinder may be mounted to one or more of the detector heads through a pivoting arm mechanism. In this configuration, the transmission sources are offset from the detector heads, and therefore offset the useful field of view (FOV).
With one or more offset transmission sources, the transmission radiation beam is offset from the center of rotation, i.e. the center of the subject, creating unsampled regions. Because information from the central portion of the subject is critical for an artifact-free reconstruction, detector heads have been shifted laterally so that the transmission fan beams cover the center of the subject. While lateral shifting of the detector heads enables transmission radiation to pass through a central region, some regions of the patient are still undersampled, and some radiation passes through the air missing the patient. In order to minimize a patient""s dose of radiation, the transmission radiation source typically generates only a limited number of radiation events per unit time. Wasting a portion of these events or rays reconstructing empty regions next to the patient is inefficient.
In order to eliminate these xe2x80x9clost raysxe2x80x9d of transmission radiation, prior art techniques concentrate on moving the patient support vertically and horizontally during data acquisition. This technique is disadvantageous because it leads to patient discomfort, especially in rapid acquisition sequences.
The present invention contemplates a new and improved contouring technique for use with transmission scans which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, a nuclear medicine gamma camera includes a rotating gantry which defines a subject receiving aperture. A plurality of radiation detector heads, which are movably attached to the rotating gantry, rotate about the subject receiving aperture with rotation of the rotating gantry about an axis of rotation. At least one radiation source is mounted to the rotating gantry such that a divergent beam of transmission radiation from the at least one radiation source is directed toward and received by a corresponding detector head positioned across the subject receiving aperture from the radiation source. A rotational drive rotates the plurality of detector heads around the subject receiving aperture and a plurality of translational drives translate independently the plurality of detector heads (i) laterally in directions tangential to the subject receiving aperture and (ii) radially in directions orthogonal to the axis of rotation. An orbit memory stores a predefined orbit which clears a subject disposed in the subject receiving aperture. A tangent calculator calculates the position of a virtual line between the at least one radiation source and an edge of a radiation receiving face of the corresponding detector head which receives transmission radiation from the at least one radiation source. A shift calculator calculates lateral and radial shifts for the plurality of detector heads such that the detector head positions are dynamically adjusted in order to maintain the virtual line tangent to an outer boundary of the subject throughout rotation of the gantry around the subject receiving aperture. A motor orbit controller controls the plurality of translational drives and the rotational drive in accordance with the orbit from the orbit memory and shift inputs from the shift calculator.
In accordance with another aspect of the present invention, a method of diagnostic imaging using a nuclear medicine gamma camera includes placing a subject in a subject receiving aperture and injecting the subject with a radiopharmaceutical. A plurality of radiation sources and corresponding radiation detector heads are positioned about the subject receiving aperture such that the radiation sources are across the subject receiving aperture from their corresponding radiation detector heads. A contour of the subject is obtained and radiation emitted by the injected radiopharmaceutical is detected using the plurality of radiation detector heads. The positions of virtual lines extending from each radiation source to an edge of a radiation receiving face disposed on each corresponding radiation detector head are calculated. The detector heads are shifted laterally such that the virtual lines are tangent to the contour of the subject. Radiation from the radiation sources is transmitted toward the corresponding radiation detector heads positioned across the subject receiving aperture and detected using one of the plurality of radiation detectors. The detected tranmsission and emission radiation is reconstructed into a volumetric image representation.
In accordance with another aspect of the present invention, a nuclear camera system includes a rotating gantry which defines a subject receiving aperture and a plurality of real radiation detector heads movably attached to the rotating gantry. The real detector heads rotate about the subject receiving aperture with rotation of the rotating gantry. A plurality of radiation sources are mounted to the plurality of real detector heads such that transmission radiation from the radiation sources is directed toward and received by the corresponding real detector heads positioned across the subject receiving aperture from the plurality of radiation sources. A plurality of virtual detector heads impose shift restrictions on the real detector heads during rotation about the subject receiving aperture. A rotational drive rotates the real detector heads about the subject receiving aperture and a pair of translational drives translate independently the real detector heads at least one of laterally and radially with respect to the subject receiving aperture. An orbit memory stores a predefined contour of a subject disposed in the subject receiving aperture. A shift calculator calculates shifts in the real detector heads according to the predefined contour of the subject and the shift restrictions imposed by the virtual detector heads. A motor orbit controller controls the translational and rotational drives in response to commands from the shift calculator.
In accordance with another aspect of the present invention, a nuclear camera includes a rotating gantry on which at least first and second detector heads are mounted. The first detector head carries an offset transmission radiation source that projects a fan bean of transmission radiation to the second detector head, where the fan beam extends between edge rays. A rotating drive rotates the rotating gantry continuously or in steps and a radial drive moves the detector heads in a radially inward direction toward a center of rotation of the rotating gantry and a radially outward direction away from the center of rotation. A lateral drive moves the detector heads with a component of motion orthogonal to the radially inward and outward directions. The nuclear camera is controlled by positioning a subject on a subject support with a region of interest at the center of rotation. A clearance offset orbit around and displaced from the subject and subject support is calculated. A subject orbit around the region of interest is calculated and the subject is injected with a radiopharmaceutical. The rotating drive and radial drive are controlled such that the detector heads are maintained tangent to the clearance offset orbit as the detector heads are rotated around the subject. The lateral drive is controlled such that one of the fan beam edge rays is maintained tangent to the subject orbit as the detector heads rotate.
One advantage of the present invention is that it maximizes the fraction of the transmission radiation beam which interacts with the subject.
Another advantage of the present invention is that it provides a full set of transmission correction data.
Another advantage of the present invention resides in that it facilitates reduction of the radiation dose.
Other benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiment.